This proposal, entitled Structural studies of leukocyte integrin interactions, aims to understand, at atomic resolution, the mechanism by which leukocyte Integrins, Mac-1 and aXB2, interact with diverse ligands via the a I domains. The major focus will be how Mac-1 binds ligands of very different categories to perform its crucial biological functions. Supported by NIH funds, in a continuous collaboration with Dr. Springer, we have successfully defined the structural basis of leukocyte integrin LFA-1 interaction with the ICAM family. We now will turn our efforts to Mac-1. Despite belonging to the same leukocyte integrin family, structurally and functionally Mac-1 is distinct from LFA-1. Expressed on neutrophils and other myeloid cells, Mac-1 binding to very diverse ligands not only mediates the leukocytes recruitment in a differential fashion from LFA-1, but also facilitates phagocytosis during inflammation and the clearance of apoptotic cells. We will study Mac-1's interaction with ICAM-1, and compare it with LFA-1/ICAM interactions to investigate their sequential engagement of ICAM-1 for leukocyte recruitment. We will study Mac-1 (and also aX(32) interactions with complement component, iC3b, the major opsonin in facilitating pathogen phagocytosis by neutrophils. We will study Mac-1 interactions with the endothelium-secreted extracellular matrix ligand, Del-1, the suggested bridge between apoptotic cells and phagocytes. We will employ several affinity-enhanced Mac-1 I domain mutants for the structural studies with these ligands' Mac-1-binding domains. We will test the hypothesis that compared to relatively flat binding interface LFA-1 associates, Mac-1's ligands may have their protruding loops to bind bumpier Mac-1 I domain. We will perform comparison of RGD-containing ligands bound to integrins with and without I domain. The investigation on the seemingly promiscuous binding ability of Mac-1 will lay a good foundation for therapeutic exploration to treat many relevant diseases.